Hi, I'm working with stepper motors and I have a question. I have found that if I use a higher voltage switching source, I get better torques from the motor. why is this?
More current is being drawn by the motor windings, therefore more torque. The controller circuit limits the current to what you have it set to. I have a stepper motor from a customer's test unit that calls for around 4 volts. The power supply in the test unit is a 42 volt unit. Their controller monitored and limited the current to the short pulses needed to turn the motor for a slurry pump.
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In the olden days a high voltage was used to get the motor to respond quicker.
For example, 36V was applied to a 12V stepper.
The stepper winding had a series resistor (power) to drop the motor voltage to 12v at steady state.
On switching, the inductance of the motor saw the full 36V but would then tapper off to 12v.
This extra 24V would kick the motor in the pants to get things going quickly.
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Of course you shouldn't apply more voltage (continuously) than the manufacturer specifies or you might burn it up. (1)
Electrical power is being converted to mechanical power so with more voltage more energy is converted to mechanical energy (or force). You probably know a DC motor normally runs faster with more voltage, and/or you can get more torque. A DC motor will also "suck" more current with a heavier mechanical load, or when it's first turned-on and is accelerating.
Electrical power is calculated as Watts = Voltage X Current. And since Ohm's Law says current is proportional to voltage (with the same load resistance), when you increase voltage the current increases and the power is proportional to the square of the voltage. (Doubling the voltage is 4 times the power).
...And if we are converting to mechanical power, 750W is about 1 Horsepower. Of course there is some inefficiency with some power lost as heat so an electrical motor that puts-out 1HP will consume more than 750W.
(1) Paul is talking of short pulses (PWM?) so the average voltage (and current) is lower.
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then it would be advisable to only increase the voltage when more torque is needed at higher RPM, to avoid generating overheating?
A stepper driver does that for you.
It works like an efficient switching buck converter, dynamically adjusting voltage/current.
Common H-bridges, like the L298, can't do that.
Leo..
Stepper coils present an inductance to the stepper driver, in addition to resistance, since voltage is not constant but is being switched on and off.
The inductive reactance is proportional to the coil inductance and also to frequency.
Torque is proportional to current, for a given motor. Rate of current rise is inversely proportional to the total reactance. Current rise follows voltage rise (lags in phase). Therefore it takes time for the current to reach its maximum value.
So, increasing voltage will help get current to the desired level quicker. But increasing speed will increase inductive reactance and also decrease current. Torque drops off with faster stepping rates.
It is well worth looking at the torque vs RPM curves of your stepper motor. Most advertise their rated torque at very low rpm, and the poorer quality motors will have the torque drop off quite rapidly with increasing speed.
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